Where are the collimating screws here?

[Ionlylookup]

Hi, I recently bought a s/h Skywatcher 150/750 reflector which didn’t come with a user manual.

I’ve noticed the focus is a little off on many objects so decided to overcome my fears and collimate it using a homemade collimating cap. 
However, I can’t seem to find the 6 screws needed for collimation as shown in various online guides. Can you pls help me understand this? 
 

also, my spotting scope gets misaligned with the slightest of a tap on it, and it’s a real PITA to fix it in dark. Have I positioned it correctly? 
 

thanks very much for your help :)

 

[Louis D]

Have you tried unscrewing the back cover plate to see if there's any adjusting screws hidden underneath?  Apologies if you've already tried this.

[Ionlylookup]

Hi Louis, I’ve not actually. I wasn’t aware that this is a back cover. I honestly thought thay these three screws are holding the primary mirror itself, is it not? 

[RT65CB-SWL]

39 minutes ago, Ionlylookup said:

also, my spotting scope gets misaligned with the slightest of a tap on it, and it’s a real PITA to fix it in dark. Have I positioned it correctly? 

If you remove the finder scope from the bracket, there maybe a groove. In this groove, normally resides an 'O' ring. If it is missing, then you can either purchase them from DIY stores or online, or use some electrical/insulation tape wound around it a few times to the desired thickness. 

[johninderby]

This could one of the SW scopes with a non adjustable fixed primary mirror . 

Edited August 9, 2020 by johninderby

[lenscap]

As Louis says, remove the cover plate and you will see 3 small Allen headed locking screws paired with 3 Phillips headed collimation adjusting screws.

The mirrors on these old blue-tube Skywatchers were usually mounted on tiny rubber O-rings (not the springs used on current models) so only minute adjustments are possible and they hold their collimation very well. My old 200p pre-dates your scope and I only tweak the collimation every couple of years,  just to keep my hand in. 😀

The critical focus distance for an F/5 scope is very short. Just applying pressure to the focus knob without apparent movement can be the difference between perfect focus and slightly off. It is best to test the focus on a high contrast target such as the lunar terminator or, in daylight, an electricity pylon. It can be difficult to judge whether a star is in perfect focus or not.

 

Edited August 9, 2020 by lenscap

[Uplooker]

As lenscap says👍

[John]

40 minutes ago, Uplooker said:

As lenscap says👍

Agreed 100%

Skywatcher uses these back plates on some of their newtonians and not others. Don't know why they put them there but they can be removed and left off to be honest with you. They actually slow down the cooling of the scope as well as getting in the way of the collimation screws.

 

 

[Ionlylookup]

Thank you everyone, you guys are absolutely brilliant. I opened the cover plate and there are indeed the tightening allen keys and adjusting screws underneath. Also, my collimation is actually little off, so have a long night ahead  

Thanks again for your quick help on this. 

[Ionlylookup]

3 hours ago, Philip R said:

this groove, normally resides an 'O' ring.

Philip, i took out the finderscope and there’s indeed a groove and a spring on the holder which fits into the groove. Though the scope still moves a bit on the slightest of touch and messes up the alignment. I’m posting the pictures below. 
Do you think the missing O ring you mentioned is the problem here? If so, could you pls point me to an online link or picture? 
thank you for your help. 
 

[John]

There should be a rubber O ring around the finder tube sitting in the narrower of the two grooves - towards the front end of the finder body.

Without it the finder simply will not stay in place in it's mount.

 

[Tim]

1 hour ago, John said:

Skywatcher uses these back plates on some of their newtonians and not others. Don't know why they put them there but they can be removed and left off to be honest with you. They actually slow down the cooling of the scope as well as getting in the way of the collimation screws.

 

 

It wouldn't make much difference when observing under typical conditions John, as you say, but in newts used for imaging, which typically have LED's, and laptops etc around, stray light entering the rear of the scope can actually go through the mirror, and appear in an image as a large donut shaped anomaly. It took me ages to figure out what was causing the issue, on an old GSO 8" I had. Since then, if using a newt for imaging, I always put a cover over the rear end.

Tim

[John]

1 minute ago, Tim said:

It wouldn't make much difference when observing under typical conditions John, as you say, but in newts used for imaging, which typically have LED's, and laptops etc around, stray light entering the rear of the scope can actually go through the mirror, and appear in an image as a large donut shaped anomaly. It took me ages to figure out what was causing the issue, on an old GSO 8" I had. Since then, if using a newt for imaging, I always put a cover over the rear end.

Tim

That might explain why the put one on the EQ mounted newtonians but not the dob's then !

Thanks Tim